Field of the Invention
The present invention is generally directed toward reverse-buckling rupture discs having reversal initiation and deformation control features on the domed portions thereof formed by laser removal of metal from the disc. The reversal initiation and deformation control features provide for exceptional control over the opening characteristics of the disc, particularly the pressure at which disc reversal is initiated, the direction in which the reversal traverses the domed portion of the disc, and the shape formed by the petal upon rupture.
Description of the Prior Art
Reverse-acting rupture discs have been employed as safety devices in process equipment where relatively low overpressure conditions, such as 7 to 10 psig, and relatively high overpressure conditions, such as 600 psig, may be encountered. While it is conventional to employ reverse-buckling rupture discs for low pressure applications, it has been found difficult to provide narrow range burst pressure tolerances at low pressures. In order to achieve reliable disc rupture at low differential pressures, disc manufacturers have incorporated structures onto the disc to provide a weakened area at which dome reversal begins under pressure conditions much lower than those required if the dome had not been intentionally weakened.
Control of the progression of the reversal event has been shown to be desirable in facilitating optimization of opening characteristics—e.g. achieving full opening under low-energy conditions, and preventing fragmentation under high-energy conditions. In addition, with proper control the energy embodied in the reversal and opening event may be used to optimize flow characteristics of the opened rupture disc. It is thus desirable to embody such control in a rupture disc design.
Toward that end, reversal initiation features (RIFs) have been incorporated into the rupture disc design, typically in the center of the dome or offset slightly away or toward from the tooth side of the dome (with reference to the tooth structure of a disc support ring positioned adjacent to the disc), so that the reversal will proceed from the RIF across the dome in a fairly balanced manner, gathering energy until the tooth is engaged.
In the past, some rupture disc designs have accomplished reversal control by using score lines as RIF's. Another structure that has been utilized previously in reversal initiation and thus burst pressure control is a dimple or other indentation formed in the dome of the rupture disc. This indentation is strategically located in a position such that the domed part of the disc will fail first at the area of depression. U.S. Pat. No. 6,494,074 discloses discs bearing indentations in the domed portion and methods of creating the indentations through mechanical deformation of the dome.
U.S. Pat. No. 6,945,420 discloses the creation of a segment in the dome of the disc having an altered metallic grain structure. Particularly, during pre-bulging of the disc blank, a portion of what will become the domed area of the disc is deflected by a post. Following this initial deflection, the post is removed and the pre-bulged disc undergoes final bulging to form the finished rupture disc. During final bulging, the indentation previously formed during the pre-bulging step is returned to its initial position with respect to the remainder of the disc body prior to formation of the indentation by the deflection post. As a result, the convex and concave portions of the disc are smooth and free of projecting surfaces or depressions. The act of creating and reversing the indentation forms a work-hardened segment in the dome of the disc having a higher residual stress than the remainder of the final bulged section. Upon encountering an overpressure condition of sufficient magnitude, the bulged portion of the disc begins to reverse at this work-hardened segment. It was discovered that altering the grain structure of a portion of the dome is effective in providing a wide range of burst pressures for a given disc thickness, particularly when the disc is formed of 316 stainless steel. However, with different metals and metal alloys, the same breadth of burst pressure range is not always observed due to the inherent crystallographic characteristics of the material.
International Patent Application Publication WO 2008/155783 discloses the use of a laser to create a region of deformation initiation. In one embodiment, the region of deformation initiation is an indentation created by heating a portion of the disc's dome and permanent set by gravity, radiation pressure, punches or other techniques. Alternatively, the region of deformation initiation is made by modification of the crystalline metallurgic structure due to the thermal variation induced by laser radiation.
U.S. patent application Ser. No. 12/331,611 describes creation of one or more non-rupturing “control scores” that together control reversal initiation. These “control scores” are positioned somewhere between the apex and the periphery of the dome, but do not proceed across the center of the dome, and are not designed to guide the deformation of the disk into an optimal shape both during and after the reversal and opening of the disc. These “control scores” are also differentiated from “rupture scores”, they are intended to not be part of the opening of the disc.
A particular problem encountered on reverse acting rupture discs of a single-petal type is the formation of a “cup” shape by the petal after rupture in liquid opening situations. The energy released during the opening event is generally very much lower than that involved in a gas-only pressure relief situation, and is not always adequate to flatten the petal back against the outlet, instead leaving the petal in the cup shape formed during the reversal. This cup shape, in turn, leads to less-desirable flow values for the rupture disc. It is therefore desirable to include functionality within the disc features that can lead to flattening of the petal, even in low-energy opening events.